Chemical Professionals

April 20, 2010

Understanding Impellers

In the last post, i discussed about use of shear / dispersion component based impellers for most of the chemical applications in place of pitch blade trubines. Based on that our readers are asking for the coverage of all types of impellers for general understanding.

Therefore, I am giving some brief details on different mixing agitators.

I am starting the post from categorization of applications where they are used to understand their selection in a better manner. There are three basic categories of mixing applications as mentioned in previous article also.

1. Gas Liquid mixing.
2. Liquid Liquid mixing.
3. Solid Liquid mixing.

Now for all of these cases, there are three basic type of agitators.

1. Axial Impellers
2. Radial or Shear Impellers.
3. Special Impellers.

So let us first start understanding Axial impellers.

Axial Impellers
In this category of impellers the major action or fluid movement is in vertical or axial direction of impeller shaft & therefore, named as axial impellers.

The major impellers in this category is a pitch blade turbine, where different types of blades, at different angles, in different blade width are used for different services. the general design is a 3 blade impeller or more approapriately a propeller, which is used.

The above design is a propeller with 3 blades similar to chemineer AP-3 and lightinin A-100 etc. However the difference is up-pumping or down-pumping. You can guess how & which one is up and which one is down pumping in nature. The up pumping means the flow goes up near the shaft of the impeller & vice-versa for the down pumping.

The power no is very low for these impellers however, it is higher than some specially designed hydrofoil shapes of these blades. So variations are as below.

All the above impellers are axial in nature with wide variation in flow characteristics, power consumption, mixing pattern etc. Now let us go to Radial Impellers.

Radial Impellers
In this category of impellers the major action or fluid movement is in the same plane of rotation or it is shearing in nature & therefore, named as radial / shear impellers.

The major impellers in this category is a rushton turbine, where different types of blades, in different blade width & shape are used for different services. the general design is a 6 blade (flat) turbine which is also more commonly known as rushton turbine.

Special Impellers
The thrid category of impellers is the special design of impellers which are used mainly for specific services mostly involving highly viscous liquids e.g. anchor, helical etc.

Continue to read this post...

March 25, 2010

Selection of Right Impeller

Its a long time since I wrote my last post in Aug'2009. Now I am retrying to find out some time out of my schedule to the benefit of all of you again. So I thought to start with this info on agitators design.

Generally in small chemical industries & particularly by different agitator designers pitch blade turbines are suggested for most of the mixing applications. But, before you select any agitator just based on the suggestion from your vendor or from some knowledge from books, re-think in terms of your process.

First & most important thing is to understand the process requirement. The process means following three major categories.

1. Gas Liquid applications.
2. Liquid Liquid applications.
3. Solid Liquid applications.

Now in most of the specilaity chemical companies the second category i.e. liquid liquid mixing is the most used application. This is the area of problem during selection of impellers, because most of the literature suggest pitch blade turbines in these cases without understanding or emphasizing on the basic chemistry & mixing need.

In my opinion, based on my experience, it is important to understand the requirement of process e.g. in case of mixing need for two liquids, it is not necessary that only homogeneous mixing is sufficient but you need to understand how the reaction rate is changing with respect to fine mixing. Means, if you use more shattering or shear during mixing does it imapct on reaction rate?

I am sure in case of mass transfer controlled reactions it is going to help a lot significantly. If you do not believe, then try changing one of your impellers & see the impact.

In such cases, the shear forces provided by flat blade turbine (FBT, Rushton) or by CD-6 turbine is much higher & very useful for increasing the reaction rate by providing fine liquid dispersion. PBT will not be my choice in this case as it does not provide any radial or shear component in mixing.

This is one example of going against well written suggestions.

Similarly in case of solid liquid mixing requirement, I am again not in favor of using PBT every time. In such cases, it depends on process behavior. if you need to break lumps or if you need to avoid formation of lumps you should have some radial mixing component along with axial component.

For example, if liquid is viscous or lump formation is to be avoided, use shear component to your advantage.

Conclusion
In my opinion mostly the shear or dispersion is helpful in case of chemical reactions, whether it is liquid liquid mixing or liquid solid mixing.

Continue to read this post...

August 23, 2009

Small Water Saving Initiative from my company

Dear All,
I just wanted to share a small initiative of my new company towards saving of water. This can be presented by you to your management for implementation which will help us to keep our globe safe and provide sufficient water savings for many families to upkeep their daily life.

The water is very important for all living things and currently it is of utmost importance for any coroporate house, individual, and governments to conserve water & promote conservation fo water.

In our present company the management has implemented waterless urinals in the corporate building. This is saving around 2 Litres of water / day / person. We have around 400 people working in this office which is therefore, saving around 2 x 400 x 300 = 240000 Litre / Year.

Now see the importance of this number. Each family (4 persons) need around 100 litre of water / day for hygeine & drinking. Therefore, it can serve for 2400 families for 1 year OR 40 families for 60 years of average life.

Continue to read this post...

August 15, 2009

Steam Properties - II

In the previous post I discussed about P & T correlation for saturated steam. I also mentioned that its better to use Excel add-in than using these formulae in Excel sheet. However, these are essentially required if you are using some non worksheet based computer program for the evaluation / simulation of your project.

So here I will cover the other major property that is enthalpy & latent heat for steam.

2. Enthalpy of Saturated Steam at Given Temperature

Enthalpy Hv = alphat + gamma

alphat = 0.99615 t + 1.8239 x 10^-6 * t^2 -0.13468 x 10^ (-0.036 t) + 0.13468

gamma = 597.34 - 0.555 t - 0.2389 x 10^alpha

alpha = 5.1463 - 1540 / T

Where t = temperature in °C
T = temperature in K

So if you need to find out the enthalpy at a given P, you first need to calculate saturation temperature based on methods given earlier.

3. Enthalpy of Saturated Water at Given Temperature

Hl = 0.001 t2 + 0.8663 x t

Where Hl = enthalpy of Saturated liquid at t temperature in Kcal/Kg
t = temperaure in °C

4. Latent heat of Vaporization at Given Temperature

Lambda = Hv - Hl

Hv & Hl are calculated above.

5. Saturation temperature from Pressure
This part is also required frequently, so I developed an equation based on data from steam tables.

t = A x y^5 + B x y^4 + C x y^3 + D x y^2 + E x y + F
Where
y = Log10 (Pv)

A = 0.03878244
B = 0.5246778
C = 2.7767678
D = 12.6450237
E = 63.9525883
F = 99.082168

Pv is in Kgf/cm2.

List of other property estimation methods on this Blog.